1. Field of the Invention
The present invention relates to a circuit breaker to be arranged in an electric device, typically in a battery pack or the like, for cutting off the flow of current when the temperature becomes higher than a predetermined temperature, and a battery pack including this circuit breaker.
2. Description of the Related Art
Devices such as battery pack and electric motor can cut off the flow of current to improve safety if the temperature becomes abnormally high. To achieve this, a circuit breaker is used which switches a contact to OFF when the temperature reaches a predetermined temperature. For example, in the case where battery packs including a lithium ion battery are charge/discharge in abnormal conditions, the temperature will be high. Accordingly, a circuit breaker is provided for cutting off the flow of current at the occurrence of an abnormally high temperature so that battery packs can be safely used. Also, the temperature may rise abnormally if electric motors are brought into an overload state or in an abnormal current state where abnormal current flows in electric motors. Accordingly, a circuit breaker is provided for cutting off the flow of current in such abnormal states and protecting electric motors so that electric motors can be safely used. A circuit breaker has been developed which used in the above applications for cutting off the flow of current in the abnormal state (see Japanese Patent Laid-Open Publication No. JP2006-100,054 A).
This circuit breaker is a flow-via-bimetal type circuit breaker which includes a bimetal 108 as moving contact metal plate 106, as shown in FIG. 1. Since the bimetal 108 serves as the moving contact metal plate 106 in this circuit breaker, there is a disadvantage that the bimetal increases the electrical resistance of the moving contact metal plate 106. In this flow-via-bimetal type circuit breaker, after the moving contact metal plate 106 of the bimetal 108 deforms so that the flow of current is cut off at high ambient temperature, when the ambient temperature decreases so that the bimetal 108 returns to the original position, the moving contact 107 contacts a fixed contact 105 again so that the circuit breaker returns to ON. If this circuit breaker is used for a battery pack, for example, although the flow of current is cut off when a battery has an abnormal temperature, the current will flow again when the temperature of the battery decreases. For this reason, the battery pack may not be safely used. In the case where a circuit breaker is used for battery packs or electric motors, and these devices are heated to an abnormal temperature, the flow of current is cut off. In the state where the flow of current is cut off, the devices are in an abnormal state. For this reason, in order to safely use the devices, it is necessary to hold the circuit breaker in the current-cutting-off state after the flow of current is cut off.
In order to solve a disadvantage of the aforementioned circuit breaker, a bimetal circuit breaker has been developed which includes a bimetal and a moving contact metal plate as separated metal plates, and additionally includes a heater for heating the bimetal (see Japanese Patent Laid-Open Publication No. JP2006-338,927 A).
FIG. 2 shows this bimetal circuit breaker disclosed in JP2006-338,927 A. In this circuit breaker, a bimetal 208 is provided between a fixed contact metal plate 204 and a moving contact metal plate 206. The fixed contact metal plate 204 includes a fixed contact 205. The moving contact metal plate 206 includes a moving contact 207 which can contact this fixed contact 205. In this bimetal circuit breaker, if the ambient temperature becomes high, the bimetal 208 will deform into an inverted shape (will bend toward the opposite direction) so that the moving contact 207 is disconnected from the fixed contact 205 whereby cutting off the flow of current. This circuit breaker additionally includes a heater 209 which heats the bimetal 208 in abnormal states. In this circuit breaker, if a protection circuit or the like detects abnormal states of a battery, current is applied to the heater by means of a switching device of the protection circuit so that the bimetal can be heated whereby cutting off the flow of current. Also, this bimetal can bend so that the flow of current is cut off if the ambient temperature becomes high. Accordingly, it is possible to cut off the flow of current without applying current to the heater if high ambient temperature is detected. In addition, in the circuit breaker which includes the heater, after the ambient temperature becomes high, this circuit breaker cuts off the current, and additionally can keep cutting off the flow of current by applying current to the bimetal (by heating the bimetal). Thus, the circuit breaker disclosed in JP2006-338,927 A can cut off the flow of current based on the ambient temperature, and the abnormal state detection detected by the protection circuit, and the like. In addition, this circuit breaker can keep cutting off the flow of current by applying current to the bimetal (by heating the bimetal).
The bimetal circuit breaker disclosed in JP2006-338,927 A bends the bimetal toward the opposite direction based on the ambient temperature so that the moving contact is disconnected from the fixed contact whereby cutting off the flow of current. In the bimetal circuit breaker, since the bimetal serves as a metal plate provided separately from the moving contact metal plate, and deforms into an inverted shape whereby disconnecting the moving contact from the fixed contact, this deformation is likely to cause time delay. The reason is that it takes time to thermally conduct the ambient temperature to the bimetal, which is arranged inside the moving contact metal plate. In such a bimetal circuit breaker, which cuts off the current based on the temperature, the circuit breaker cuts off the flow of current when an abnormal temperature is detected. Accordingly, the circuit breaker is required to cut off the flow of current as soon as possible when the ambient temperature rises to a predetermined temperature. That is, if deformation of bimetal into an inverted shape is delayed when the ambient temperature quickly rises, the battery or the electric motor may be damaged. This problem can be prevented by setting the deformation temperature of bimetal lower. However, in the case where the bimetal circuit breaker cuts off the flow of current at a lower temperature, there is a problem that the flow of current will be cut off even if the battery or the electric motor is used for a long time in a proper state (e.g., where the battery or the electric motor operates without adverse thermal influence). In addition, since the bimetal is deformed into an inverted shape by thermal expansion difference between laminated metal layers of the bimetal, it is difficult to construct a bimetal which deforms into an inverted shape at lower temperature. From this viewpoint, for the cutoff temperature in which the circuit breaker actuates and cuts off the current of the battery or the electric motor, it is important to set the cutoff temperature as high as possible in the range where no problem arises but lower than the temperature where a problem arises. Accordingly, the circuit breaker is required to have the characteristic which quickly cuts off the flow of current at a temperature lower than the temperature which damages the device.
It is a first object of the present invention to provide a bimetal circuit breaker which has the above characteristic, that is, which includes a moving contact metal plate and a bimetal as separated metal plates whereby reducing the ON resistance, and can quickly cut off the flow of current when abnormality arises, and a battery pack which includes this circuit breaker.
On the other hand, there is a disadvantage that the thickness of a bimetal circuit breaker will be large because the breaker includes a moving contact metal plate and a bimetal as separated metal plates (i.e., whereby reducing the ON resistance), and additionally includes a heater for heating the bimetal, as shown in FIG. 2. Since bimetal circuit breakers are often built in devices such as battery pack, it is required to reduce the thickness of bimetal circuit breakers so that bimetal circuit breaker can be built in even a small device.
It is a second object of the present invention to provide a bimetal circuit breaker which has the above characteristic, that is, which has a small thickness as a whole, and can quickly cut off the flow of current based on the ambient temperature, and a battery pack which includes this circuit breaker.